Rehoboam and Jeroboam were continually at war with each other throughout Abijah’s lifetime.

(5:1) Solomon ruled all the kingdoms from the Euphrates River to the land of the Philistines, as far as the border of Egypt. These kingdoms paid tribute as Solomon’s subjects throughout his lifetime.

I will not take the whole kingdom from his hand. I will allow him to be ruler for the rest of his life for the sake of my chosen servant David who kept my commandments and rules.

He did this because David had done what he approved and had not disregarded any of his commandments his entire lifetime, except for the incident involving Uriah the Hittite.